Motor control having a feedback stablilized generator

ABSTRACT

A variable voltage motor control wherein the hoist motor armature is supplied from a dynamoelectric machine having its characteristics linearized by a closed negative feedback loop including a controlled supply for its shunt field. The effective constant gain of the generator enhances operation particularly in elevator hoist motor controls.

United States Patent Inventor Richard C. Loshbough Toledo, Ohio Appl.No. 757,929

Filed Sept. 6, 1968 Patented Feb. 2, 1971 Assignee Reliance ElectricCompany Euclid, Ohio a corporation of Delaware MOTOR CONTROL HAVING AFEEDBACK Primary ExaminerOris L. Rader Assistant ExaminerK. L. CrossonAn0rneyWilson & Fraser STABLILIZED GENERATOR znrawing ABSTRACT: Avariable voltage motor control wherein the US. Cl 318/146, hoist motorarmature is supplied from a dynamoelectric 318/156, 318/158 machinehaving its characteristics linearized by a closed nega- Int. Cl l-l02p5/22 tive feedback loop including a controlled supply for its shuntField of Search 318/ 145, field. The effective constant gain of thegenerator enhances 146, 156, 158 operation particularly in elevatorhoist motor controls.

22 2 a 2.9 3 3 32 39 21 l9 l8 l3 LAG-LEAD CONTROLL GEN FWITERNCQPENSATDN SOURCE 23 31 as M 35 I5 17 12 25 CAR ll MOTOR CONTROL HAVINGA FEEDBACK STABLILIZED GENERATOR CROSS-REFERENC E TO RELATEDAPPLICATIONS This control is an improvement upon the elevator hoistmotor controls disclosed in US Pat. No. 3,435.9l6 issued April 1, I969on application Ser. No. 373,136 by Robert E. Bell. Donivan L. Hall andRichard C. Loshbough filed June 4. 1964 and entitled "Elevator MotorSpeed Control Including High Gain Forward Loop and Lag-LeadCompensation" and divisions thereof, and to such controls as disclosedin application Ser. No. 380,385 filed July 6, 1964, now US. Pat. No,3,523,231 for "Elevator Control" in the name of Donivan L. Hall, RichardC. Loshbough and Gerald D. Robaszkiewicz.

SUMMARY OF THE INVENTION This invention relates to improvements inelevator hoist motor control wherein velocity or position based feedbacksignals are combined with pattern signals to produce an error signalwhich is amplified, with suitable compensation, for controlling theshunt field of a generator supplying hoist motor armature current. Moreparticularly, the invention relates to linearizing the generator shuntfield characteristics by means of a closed negative feedback loop fromthe armature to the controlled supply to the generator shunt field.

The loads imposed on elevator hoist systems, the variations in ambientconditions for those systems, and the exacting requirements as tocontrol of a car position have dictated the use of variable voltagehoist motor controls employing generators having special characteristicsparticularly as to their compounding, changes with temperature, andmagnetic hysteresis. Thus an elevator hoist motor system mustaccommodate hauling, balanced and overhauling loads for operating upwardand downward. Flat compounding of the generator and internalcompensation have been achieved in both velocity and position basedcontrols by employing closed negative feedback loops having high gainand suitable compensation whereby the tendency toward deviations fromdesired characteristics are largely swamped out. However, even withthese expedients the approach of the generator to saturation causedvariations in characteristics which dictated a generator which operatedwell down on its saturation curve and offered relatively linearcharacteristics. The present invention enables a conventional commercialgenerator of a capacity corresponding to the maximum power requirementsof the drive to be utilized in an elevator hoist motor system by forcingthe generator to appear to have a linear characteristic over its entirerange.

In elevator systems of conventional form, the band width varies with theoperating point on the generator saturation curve, hence if theunbalanced load is large, during acceleration or deceleration theresponse of the car to a change in command signal tends to lag thatsignal as an inverse function of band width, An objective in slowing anelevator car is to have a predetermined car speed for all operatingconditions are certain points given distances from the landing at whichthe car is to be stopped. Variations in loading alter the point on thegenerator shunt field operating curve at which the system operates suchthat a slowdown of an ascending car fully loaded is aided by the loadingso that the .voltage at the shunt field is at or near zero and a highgain is available while the slowdown of an empty ascending car requiresthe generator to provide a retarding force so that a high voltage ispresent at the shunt field and a low gain is exhibited. This drop ingain, i.e. the voltage in voltage out" slope may amount to a factor oftwo or three as the generator approaches saturation and the resultantdecrease in band width will increase the lag in motor response to thechanges in the pattern signal so the that the speed at leveling pointsvaries during the approach to the floor and the car either spots (stopsshort of the landing), overshoots or at least varies in its motion toadd unwanted jerk "and acceleration components.

In the present invention an inner negative feedback loop is closedaround the generator armature terminals and provided with sufficientgain to force the generator to have a linear characteristic. This hasenabled superior control of the hoist motor system to be achieved with agenerator having a capaci- LII ' the time consuming cut and trytechniques previously required. As a result, an average saving per hoistmotor installation of the order five hundred dollars has been realizedby taking advantage of the generator linearization of this inventionwhile improving the operating characteristics of the systems.

An object of this invention is to improve variable voltage elevatorhoist motor controls.

Another object is to reduce nonlinearity in the generator shunt field ofa variable voltage elevator hoist motor control to a point where it isnegligible.

A third object is to enhance the operating characteristics of variablevoltage elevator hoist motor controls.

A fourth object is to reduce the initial and maintenance expense ofvariable voltage elevator hoist motor controls. In accordance with theabove objects one feature of this invention is the incorporation of aninner negative feedback loop around a dynamoelectric generator in avariable voltage elevator hoist motor control. A further feature is theprovision of a negative feedback path having a gain of the order of 0.02from the generator armature terminals to a summing point for thegenerator shunt field control and amplification means within the loop toprovide a loop gain of the order of 10.

DESCRIPTION OF THE DRAWINGS The above and additional objects andfeatures will be more fully appreciated from the following detaileddescription when read with reference to the accompanying drawing inwhich;

FIG. 1 is a block diagram of a velocity servo-type variable voltageelevator hoist motor control including the inner negative feedback loopof this invention; and

FIG. 2 is an expanded diagram partially in schematic and partially inblock form illustrating in more detail the application of the inventionto a commercial embodiment of the control of FIG. 1.

DETAILED DESCRIPTION Elevator car lll and its counterweight 12 have atraction drive from hoist cable sheave 13 through cable 14. Hoist motor15 drives sheave 13 through shaft l6 to which is also coupled tachometer17. Control of the motor 15 is by control of the generator 18 supplyingits armature with a variable voltage dependent upon the power suppliedto the shunt field winding 19 for the generator. Motor 15 has aseparately excited shunt field (not shown) having a constant excitationlevel while the car is set to run. Generator shunt field 19 is supplied,for example, by controlled rectifiers having phase control firingcircuits as shown in the above mentioned Bell et al. Pat. application,as represented by controlled source 21.

Control of the controlled source 2! is by means of an amplified andcompensated error signal derived from the deviation of actual motorspeed from commanded motor speed. A velocity servoloop having negativefeedback reduces the error to a small value by employing high gain withcompensation appropriate for an elevator system wherein the closed loopgain is reduced to a value less than unity at and above the naturalresonant frequency of the resonant circuit comprising the totalinductance and resistance in the hoist motor armature circuit and thecapacitive effect of the total driven mass including the car, thecounterweight and the driving means-for the car, coupled into theannature circuit through the hoist motor. The total loop gain in theexample is about 30 with lag- A velocity servo control is shown inFIG. 1. A commanded velocity pattern generator 22 is arranged to developa signal for a full speed run having a gradually increasing value fromzero speed to maximum acceleration, a constant acceleration. a gradualtransition from a maximum acceleration to zero acceleration at a maximumvelocity, where full speed run is to be made, and a deceleration patternwhich approaches a mirror image of the acceleration. The signal for ashort run, when full speed is not achieved, includes the gradualacceleration to maximum acceleration and initiates a breakover forgradually reduced acceleration followed by gradually increaseddeceleration to maximum deceleration until zero velocity is approachedwhen a gradual decrease to zero acceleration is made at zero velocity.These velocity pattern signals represented as voltage as a function oftime are applied from output 23 of the pattern generator to summingpoint 24. Tachometer 17 developes a voltage proportional to actual hoistmotor speed and a network 26 having a feedback constant K,, whichadvantageously is set to provide 3 volts/100 ftJminute of rated maximumcar speed to applies the speed signal to lead 27 and summing point 24.

An error signal, the algebraic sum of the commanded speed pattern andfed back tachometer signal which is a function of actual speed, issueson lead 28 from summing point 24' to the lag-lead compensator 29.Amplifier 31 amplifies the signal for application to summing point 32through lead 33.

Linearization of the generator so that it appears as an ideal generatoris accomplished by an inner negative feedback loop 34 around thegenerator. This loop includes a lead 35 from the generator armatureterminals to the network 36. From network 36, lead 37 extends to summingpoint 32. The net signal resulting from the algebraic sum of signals atleads 37 and 33 if fed on lead 38 to amplifier 39 and thence tocontrolled source 21 for the generator shunt field 19. An inner loopgain of about has been found to be satisfactory-in that it is sufficientto accommodate excursions over a substantial portion of the operatingcharacteristic, as where the gain is reduced by three from one extremeto the other, and yet the inner loop is not so underdai'nped as torequire compensation. With a loop gain of 10 the inner loop as has thecharacteristic of a slightly underdamped quadratic.

The network in the feedback path of the inner loop is chosen to have again of 0.02 and the summing circuit has a gain of 0.5 so that with again of approximately 16 in the amplifier 39, a gain in the controlledsupply of approximately l5 and a generator voltage gain (field toarmature) of approximately four the inner loop gain is about 10.

The techniques combining the elements of FIG. 1 are disclosed in greaterdetail in FIG. 2 for a commercial embodiment of an elevator hoist motorcontrol. The pattern generator produces a voltage proportional tovelocity which is positive at lead 41 relative to lead 42 in the formtypified at 43. An ascending car has pattern input 23 positive and adescending car has it negative by the switching matrix 44. Up relay Uand down relay D (not shown) control the polarity of the pattern at 23in response to signals corresponding to the conventional directionalizedgenerator field relays such that when car motion is to be initiatedupward and during such motion, relay U is energized and for down travelrelay D is energized under the same conditions. When the car is stopped,relays U and D are dropped out to close their back contacts in matrix 44thereby shorting the output leads 41 and 42 for the pattern andgrounding lead 23. For up travel, relay U is energized to couple throughits front contacts U lead 41 to the summing point 24 and to couple lead42 to ground while the back contacts U open the short of 41 and 42andthe ground to 23. For down travel, front contacts D couple patternoutput lead 42 to 23 and output lead 41 to ground while the backcontacts D open the short of 41 and 42 and the ground'to 23.

potentiometer 46 is adjusted to provide 3 volts at lead 27 for each feetper minute of car speed for a tachometer which generates a voltage inthe range of 6 to 12 volts per 100 feet per minute of car speed.

Velocity commanded by the pattern and actual velocity are compared orsummed differentially at 24 by application of the voltage on leads 23and 27 through resistors 47 and 48 of equal magnitude. The net voltagerepresenting the velocity error or the amount by which the actualvelocity differs from the commanded velocity is compensated andamplified for control of the firing of the silicon controlled rectifiers49 and 51 by lag-lead compensator 29 and operational amplifier 31. Thiscompensator and amplifier'can be of the type generally shown in theabove noted Bell et al. application. A single laglead network made up'brresistor and capacitors (not shown) and having a lead break frequency offrom 2.5 to 5 radians per second has been found'satisfactory for theelevator systems to which the invention has been applied.

The gain employed in the operational amplifier 31 is established by theparameters of the system. For a typical system it is of the order of theclosed loop gain, i.e. 30, and is established by the setting of rheostat52. An increase in resistance increases the gain and causes the systemto reduce its error. The gain setting is'chosen based upon the practicallimits for a comfortable elevator ride with'satisfactory stability inthe system for the parameters and constraints employed. Reducing thegain by decreasing the rheostat setting slows the response.

A running relay, AM, not shown is picked up when the elevator is readyto run and droppedout after the car has been stopped. While the car isstopped at a landing back contacts of relay AM close to ground lead-28thereby forcing the apparenterror signal to the compensator 29 to zero.At this time the gain of amplifier 31=is reduced by the shunting ofresistor 53 with resistor 54through closed back contact AM.

With the car stopped and the feedback signal removed'due to the closedAM back contact grounding lead 28, small zero offsets in the amplifierscan become significant since there is no correction signal available.Such small signals are notsignificant to indicate faulty operation. Thereduction in the gain of amplifier 31 by the shunting of resistor 53maintains the sin signals resulting from such zero offsets at levelsineffective to affect the monitoring circuits of the system and therebyfalse indications of unsafe operating conditions.

The velocity amplifier output signal is applied to summing point 32through resistor 55 as a voltage of the same polarity as applied fromthe pattern signal switching matrix of lead 23. This signalisproportional to desired voltage on the generator armature 18. Armaturevoltage for the generator is attenuated at'resistor 56 and groundedresistor 57 and applied through front contact AM and resistor 58 tosumming point 32. Re sistors 59 and 61 prevent application of fullarmature current to the control circuitry; thereby affording shortcircuit protection, and resistors 56 and 57 are chosen to scale armaturevoltavoid age down by a factor of 50 to l, effectively negating theeffect of resistors 56 and 57 because of therelative values ofresistance. The voltage at lead 37 is of opposite polarity to that atlead 33 so that it is negative for up and positive'for down' direction.The closed inner loop wipes out effects of generator iron saturation andIR drop through comparison of the actual generator armature voltage andthe desired generator arma ture voltage and the amplification of theerror signal tending to force that signal to a negligible value in anamount which is a function of the loop amplification.

During the interval the car is set to run, front contact A, AM is closedtoclose-the inner loop 34, however when the car is stopped and thesuicide connection'of the fields is made it is desirable to open thisloop to avoid any feedback. Thus con force the error toward 'zero and atlead 64 imposes the resultant signal on a phase control firingcircuit-65forthegates of SCR'S 49 and 5 l The net output of operationalamplifier 39 and its emitter followers 62 and 63 as it appears at 64 islimited in its rate of change through the operation of the phase shaftnetwork comprising capacitance 66 and resistor 67 to avoid excessivecirculating currents should an abrupt change in signal be experienced.Transistors 62 and 63 offer unity gain and the gain of operationalamplifier 39 is determined by the relationship of the value ofresistance 68 to that of resistance 69. Minor changes on the output 64are not in fluenced by the capacitance 66 and resistance 67. Abruptchanges of signal pass through the RC filter 66 and 67 and therefore arenot reflected at output 64.

The effect of resistance 67 and capacitance 66 on the phase shift in thesystem is negligible in view of the high gain around the loop ofamplifier 39 and its emitter followers 62 and 63 as determined byresistor 71. This gain moves the break point of the network well beyondthe frequency range of interest in elevator applications, hence anytendency to introduce a lag in the system is of no effect on itsoperation. The limited rate of change of gate control 65 prevents areversal of the net current in intervals of less than one cycleduration, thereby protecting the choke 72 and the capacitance 73connected across the SCRs 49 and 51 and across the highly inductivegenerator shunt field l9.

Alternating current is supplied to the SCR anode cathode circuits atterminals 74 to which the primary of transformer 75 is connected. Thesecondary of transformer 75 is coupled through lead 76 to the anode ofSCR S1 and the cathode of a diode 77 shunting SCR 51, and through choke72 and the capacitance 73 to the anode of SCR 49 and the cathode of itsshunting diode 78. As the gate control applies a gating potential to thecontrol electrodes of SCR, it fires to pass the remainder of the halfcycle of alternating current. A steady state level of DC is thusdeveloped across the capacitance 73 and choke 72 and is applied acrossgenerator shunt field 19 to control the generator. The impedance ofcapacitance 73 and choke 72 to 60 cycle alternating current is low, ofthe order of 2 ohms, so that any abrupt change in the gating pattern ofthe SCRs which would result in a shift from firing SCR 49 over asubstantial portion of the halt cycle it is forward biased, to firingSCR 5] over a substantial portion of the half cycle it is forward biasedin effect reverses the current in the low impedance shunt withpotentially destructive efi'ect.

Back to back rectifier 79 can be a GE. Thyrestor diode providing surgeprotection. In accordance with the aforenoted Bell et al. application,the hoist motor control system can be provided with safety means toeither shut down the system or viewed across capacitance 73 whichexceeds a operate it at safe speeds when the output to the generatorshunt field exceeds safe levels or when the velocity error signal isexcessive as indicated by the EXCESS AMPLIFIER OUT- PUT SAFETY or EAOcircuit 81 and the EXCESS ERROR SAFETY or EE circuit 82. As implied bythe names, the EAO circuit 81 is set to respond to a signal to the shuntfield as safe operating speed by responding to the threshold of such asignal level by shutting down the hoist motor operation through theopening of the main switch for the hoist motor. A similar shutdown isinstituted when the velocity error signal exceeds safe value establishedby a threshold signal level preset in EE safety at 82. This error issensed at summing point 83 through summing resistors 84 and 85 connectedin parallel with summing resistor 47 and 48 connected to the patterngenerator and tachometer 17 respectively.

When any safety is operated it requires a manual reset, wherein themotor generator set (not shown) is stopped, before the system canoperate again.

The above system disclosure is illustrative and not to be read in alimiting sense. Generator linearization can be used advantageously in asystem employed a positional pattern and feedback without departing fromthe invention and other adjuncts can be incorporated in the system ascontemplated.

lclaim:

l, A motor control for a direct current motor comprising a motorarmature, a direct current generator, a shunt field for said generator,an armature for said generator, means operative coupling said generatorarmature to said motor armature, means to generate a voltage which is afunction of actual motor speed, means to generate a voltage which is afunction of commanded motor speed, means differentially summing saidcommanded and actual motor speed voltages to develop a motor speed errorsignal, means generating a commanded generator armature voltage inresponse to said motor speed error signal, means sensing actualgenerator-armature voltage, means differentially summing said commandedand actual generator-armature voltages to develop a generator-armatureerror voltage, an amplifier between said motor speed signal summingmeans and said generator-amiature voltage signal summing means, acontrollable source of electrical energy connected to supply saidgenerator shunt field, means controlling said controllable source inresponse to said error voltage, and means to reduce the gain of saidamplifier while said motor is stopped and to increase the gain of saidamplifier while said motor is set to run, whereby zero offset of saidamplifier is minimized when said motor is stopped.

1. A motor control for a direct current motor comprising a motorarmature, a direct current generator, a shunt field for said generator,an armature for said generator, means operative coupling said generatorarmature to said motor armature, means to generate a voltage which is afunction of actual motor speed, means to generate a voltage which is afunction of commanded motor speed, means differentially summing saidcommanded and actual motor speed voltages to develop a motor speed errorsignal, means generating a commanded generator armature voltage inresponse to said motor speed error signal, means sensing actualgenerator-armature voltage, means differentially summing said commandedand actual generator-armature voltages to develop a generator-armatureerror voltage, an amplifier between said motor speed signal summingmeans and said generator-armature voltage signal summing means, acontrollable source of electrical energy connected to supply saidgenerator shunt field, means controlling said controllable source inresponse to said error voltage, and means to reduce the gain of saidamplifier while said motor is stopped and to increase the gain of saidamplifier while said motor is set to run, whereby zero offset of saidamplifier is minimized when said motor is stopped.